The integration of lattice-mismatched layers on conventional substrates enables the fabrication of numerous electronic and optoelectronic devices on standard substrates. An obstacle in the integration of such materials onto conventional substrates, such as bulk silicon, is the lattice mismatch between the deposited layers and the underlying conventional substrate. Lattice mismatch implies that the equilibrium lattice constant of the material being deposited differs from the lattice constant of the substrate, leading to the creation of defects to accommodate the mismatch. Examples of such defects include misfit dislocation segments along the interface of the deposited lattice-mismatched layer and the substrate, in addition to threading dislocation segments, connected to the ends of the misfit segments and terminating at the surface of the lattice-mismatched layer. The presence of these threading dislocations in device layers may degrade device performance and complicate processing; hence the minimization of threading dislocation densities in lattice-mismatched layers is of importance in the fabrication of electronic and optoelectronic devices.
A number of techniques have been employed to reduce threading dislocation densities in lattice-mismatched layers. A common technique includes the deposition of an alloy graded layer on the substrate, wherein the content of the alloy is gradually increased so as to suppress the nucleation of large numbers of threading dislocations. For example, in the case of silicon germanium on silicon substrates, a silicon germanium graded layer may be deposited on the substrate, wherein the germanium content of the silicon germanium alloy is gradually increased up to a final germanium content desired for the application. The end result is a relaxed (i.e., unstrained) alloy graded layer, possessing an equilibrium lattice constant differing from the substrate lattice constant, wherein the threading dislocation density at the surface of the graded layer is several orders of magnitude lower than if a uniform composition alloy layer, possessing the final alloy composition, had been directly deposited onto the substrate.